The present disclosure relates to a urea SCR system.
Japanese Laid-Open Patent Publication No. 2015-001208 describes a known example of a urea selective catalytic reduction (SCR) system serving as an exhaust purification device that purifies exhaust by detoxifying nitrogen oxide (hereinafter referred to as “NOx”). The urea SCR device uses a urea water adding device including an injector that injects urea water to exhaust gas and a selective reduction catalyst located downstream of the position where the urea water is added. In the urea SCR system, the urea water added to exhaust gas is hydrolyzed to ammonia by the heat of the exhaust gas. When the exhaust gas containing the ammonia flows into the selective reduction catalyst, NOx in the exhaust gas is reduced to nitrogen and water by the ammonia serving as a reducing agent.
In the urea SCR system, urea contained in urea water may crystalize. In some cases, this causes the injector to be stuck closed. When the injector is stuck closed, it will be difficult to add urea water to exhaust gas. Thus, there is a demand for determining with high precision whether or not the injector is stuck closed. Additionally, there is a demand for a technique for quickly overcoming stuck closing of the injector that results from crystallization of urea.